@Koli,
Because it's creepy and stalkerish. What if she was allergic, and you put her into anaphylactic shock? What if she was dieting (never mind if you think she needs to or not) and you killed off her willpower? What if she had to explain to some other guy why suddenly there's chocolate in her lunchbox?
Do not touch other people's stuff without permission.
Didn't you learn that when you were two years old?
It still applies, even if you want to have a romance with someone.
Ugh. Creepy, stalkerish, and potentially dangerous. Way to go, Romeo.